ABSTRACT HIV-1 is primarily mucosally transmitted. The human HIV-1 vaccine trials have taught us important lessons about vaccine vectors, immunogen design, and immune responses important for protective efficacy. Yet, none of the human HIV-1 vaccine trials tested whether a mucosal route of immunization could contribute to protection against HIV-1 acquisition at the portals of entry. Despite some unique challenges that mucosal vaccines present, several oral vaccines against infectious diseases have been successfully developed and licensed. The success of oral vaccines is based on the rich regional lymphatic network of the Waldeyer's Ring that provides an easily accessible portal for oral vaccine uptake, and, as an intrinsic part of the systemic lymphatic network, enables the induction of local and systemic protective immune responses. An oral HIV-1 vaccine would be particularly relevant for the prevention of mother-to-child-transmission of HIV-1. Despite steadily increasing access to antiretroviral therapy, the negative cascade effect from HIV-1 diagnosis of pregnant women to continued treatment throughout the breast-feeding period and test-and-treat for the infant persists. In 2014 alone, 200,000 new pediatric HIV-1 infections occurred, the majority by breast milk transmission. Breast-feeding remains essential to provide passive immunity and nutrition to newborns in resource-poor countries where poorer responses to standard pediatric vaccines have been linked to differences in commensal flora between breast-fed and formula-fed infants and nutritional deficits. The impact of the microbiome on vaccine efficacy is further supported by data that the initial HIV-1 antibody response in adults is shaped by cross-reactive antigens of commensal bacteria in the human ileum. The contribution of the oral microbiome though remains largely unknown. We hypothesize that the intricate interplay between the oral mucosa, the oral microbiome, and oral mucosal immunity in the context of the infant's immune development will impact the efficacy of a pediatric oral HIV-1 vaccine Here, we propose to test a sublingual (SL) MVA-SIV prime/ SL MVA-SIV plus SIV Env protein boost regimen for protection against repeated low-dose oral SIVmac251 challenge in infant macaques. The locally defined SL route will ensure oral uptake, and ease of application would translate into high acceptance within the pediatric population. To enhance local immunity at oral SIV entry sites, the SIV Env protein will be applied with a novel, clinically relevant mucosal adjuvant, the double mutant of the heat labile enterotoxin, dmLT. The comparison of vaccine efficacy in in breast-fed and formula-fed infants will provide proof-of-concept that the oral microbiome alters local and systemic immune responses and thereby, impacts oral SIV vaccine efficacy.